1. Field of the Invention
The present invention relates generally to a Multiple Input Multiple Output (MIMO) system. More particularly, the present invention relates to an apparatus and method for scheduling a multiuser and a single user.
2. Description of the Related Art
A Multiple Input Multiple Output (MIMO) scheme, which allocates a plurality of data streams to a plurality of users, is a Spatial Division Multiple Access (SDMA) scheme capable of achieving Multiuser Diversity (MUDiv) by allocating a data stream to a user with optimal channel condition using a spatial domain.
Of conventional technologies, Per User Unitary feedback/beamforming and Rate Control (PU2RC) is the only technology that enables Spatial Multiplexing (SM) for allocating several data streams to the same user and the SDMA for allocating several data streams to multiple users at the same time. Herein, it is defined that the SM is a Single-User MIMO (SU-MIMO) scheme and the SDMA is a Multiuser MIMO (MU-MIMO) scheme.
The SU-MIMO scheme maximizes a peak data rate by using an advanced receiver technique such as Successive Interference Cancellation (SIC). The MU-MIMO scheme maximizes the overall system throughput by acquiring the MUDiv by allotting several data streams to a user that exhibits a maximum capacity through competition with the multiple users.
The MU-MIMO scheme aims at a different purpose than the SU-MIMO scheme but also uses the different Channel Quality Information (CQI) feedback as fundamentally required by the closed loop MIMO scheme. The MU-MIMO scheme feeds back the CQI according to a feedback scheme when each user calculates a precoding matrix based Signal-to-Interference and Noise Ratio (SINR) based on a Minimum Mean-Squared Error (MMSE) receiver technique. By contrast, the SU-MIMO scheme calculates and feeds back the stream SINR based on a SIC receiver technique.
Different kinds of CQI feedback methods include a full CQI feedback, a partial CQI feedback, and a reduced CQI feedback. Regarding the full CQI feedback method, when the transmitter uses G-ary precoding matrixes, the receiver calculates SINR of acquirable streams with respect to every precoding matrix and feeds back every SINR to the transmitter. The full CQI feed back method is impracticable because it must process too much feedback information. The partial CQI feedback method feeds back the preceding matrix index of the maximum sum rate of the stream SINR among the calculated stream SINRs of the precoding matrixes, and the stream SINR acquirable for the corresponding preceding matrix. The partial CQI feedback achieves a practical tradeoff between throughput and redundancy. The transmitter groups feedback information received from the users to the same preceding matrix and selects a user who has fed back the best stream SINR of each precoding matrix, to thereby allocate the corresponding streams to users that maximize the stream SINR sum rate acquirable for the corresponding precoding matrix. The reduced CQI feedback method feeds back to the transmitter the preceding matrix index of the best sum rate of the stream SINR, the index of the stream index acquiring the best SINR of the stream SINRs of the corresponding precoding matrix, and the SINR acquirable at the receiver using the preceding matrix and the stream. The reduced CQI feedback method requires the least CQI feedback amount and its performance is as good as the partial CQI feedback when the number of users is large. Hence, the reduced CQI feedback is the most practical CQI feedback scheme of the SDMA.
In the mean time, if the MIMO system adopts conventional SU-MIMO and MU-MIMO to schedule a multiuser and a single user, a static SU-MIMO and MU-MIMO switching scheme and a dynamic SU/MU-MIMO scheme are available.
The static SU-MIMO and MU-MIMO switching scheme performs only SU-MIMO or MU-MIMO in general by taking into account the channel condition of the upper layer and the number of users. While the static SU-MIMO and MU-MIMO switching scheme might be the simplest scheme, the overall efficiency is prone to deteriorate because neither the MU-MIMO nor the SU-MIMO are executable, and thus the optimization for maximizing the system capacity is not obtained.
In contrast, according to the dynamic SU/MU-MIMO scheme which is contrary to the static switching scheme, every user calculates suitable CQI on the assumption that it will be serviced using MU-MIMO (SDMA) or SU-MIMO (SM), and then feeds back the calculation result to the transmitter. In doing so, every user feeds back (the reduced CQI feedback) the precoding matrix and the stream index highly preferred for the SDMA service and the SINR acquirable at the receiver when the transmitter uses the precoding matrix and the stream, and feeds back (the partial CQI feedback) the stream SINRs calculated for the SM service with the SIC. The transmitter determines which is better to service using SU-MIMO or MU-MIMO with respect to the allocated chunks, and allocates the chunks so as to maximize the system capacity. The dynamic SU/MU-MIMO scheme can also maximize the system capacity by allotting one of the MU-MIMO and the SU-MIMO, which maximizes the sum rate, to the user. However, since each user needs to calculate and feed back both the reduced CQI feedback for the MU-MIMO and the partial CQI feedback for the SU-MIMO, the dynamic SU/MU-MIMO scheme is subject to the shortcoming that the required CQI feedback amount is considerable. In addition, if the dynamic SU/MU-MIMO scheme is expanded to Orthogonal Frequency Division Multiplexing (OFDM), N times more feedback overhead is required linearly.